A gear or gear train has been used as a means for transmitting rotary motion so far. Such gear or gear train transmits rotary motion not at variable angular velocity ratios depending on the number of gear teeth but at an fixed angular velocity ratio. To solve this problem, Japanese Unexamined Patent Publication No. (Patent Kokai No.) 10-184852 (1998) discloses a transmission device that can transmit rotary motion at variable angular velocity ratios. However, a gear disclosed in that publication is an odd gear, so that it is difficult to produce it by machining, and the production of gear requires a great deal of production cost and time. To solve this problem, Japanese Unexamined Patent Publication No. (Patent Kokai No.) 2000-81112 discloses a gear and gear train having a plurality of tooth profiles, each of which comprises a semicircular top curved line and a semicircular bottom curved line and in which a distance between a pitch line and a rotation center is not constant.
However, there has emerged a need for developing a more practical gear which has high durability to withstand a load, which does not make noise, which can be produced without problems, and in which no unnecessary force is applied to a rotation shaft.
As the result of our intensive studies on odd gears, we have eventually found the present invention.